Method for recovering gaseous nitrogen from the waste nitrogen stream of an air separation unit

ABSTRACT

The method for the production of nitrogen can include the steps of a) providing a waste nitrogen gas stream at a first pressure called low pressure, wherein the waste nitrogen gas originates from a low pressure column of a cryogenic air separation unit; b)compressing the waste nitrogen gas stream to a second pressure in a waste nitrogen compressor to produce a pressurized nitrogen stream; and c) introducing the pressurized nitrogen stream to a nitrogen generator under conditions effective for producing a purified nitrogen product, wherein the purified nitrogen product has a higher concentration of nitrogen as compared to the pressurized nitrogen stream.

RELATED APPLICATIONS

This application is a non-provisional application of U.S. Provisional Applicant No. 62/361,823, filed Jul. 13, 2016, which is herein incorporated by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a distillation separation process and plant. In particular, it relates to a process and an apparatus for production of a gaseous nitrogen stream having a high purity with reduced capital investments.

BACKGROUND OF THE INVENTION

The usual solution to provide a gaseous nitrogen stream with a high purity is to purify air. The process is based on fractionated air distillation at cryogenic temperature in a single column. It involves five steps: compression, purification, heat exchange, distillation and cold production.

EP-A-0299364 discloses a process in which a nitrogen-rich gas flow from the head of the medium-pressure column of a double column is fed to a column having a head condenser and a flow of high pressure nitrogen is withdrawn at the top of this column.

U.S. Pat. No. 5,934,106 discloses an air separation apparatus for the production of nitrogen in a single column having a top condenser. The vaporized rich liquid is expanded in a turbine having the same inlet temperature as the air turbine and the vaporized rich liquid turbine is coupled to a cold vapor compressor from an intermediate level of the single column.

GB-A-2126700 discloses an air separation apparatus for the production of nitrogen in a single column having a top condenser. According to one mode of operation, the vaporized rich liquid is expanded in a turbine having the same inlet temperature as the air turbine and the relaxed air and the vaporized rich liquid relaxed are sent to air.

On air separation units which are not intended for the production of pure nitrogen, it is of course not possible to make nitrogen gas and/or pure liquid a posteriori without making very significant changes to devices.

Therefore, the methods known heretofore fail to provide a simple and economical way of producing pure liquid and/or gaseous nitrogen from a nitrogen-enriched gas from a main air separation apparatus.

SUMMARY OF THE INVENTION

The present invention is directed to a process that satisfies at least one of these needs. In certain embodiments of the invention, a method is provided for increasing the nitrogen production of an existing air separation apparatus by using at least a portion of the waste nitrogen as a feedstock for a separate cryogenic distillation system.

In a typical air separation unit, a stream of dry enriched nitrogen (typically above 90%) is coming out of the cold box and is usually used for the regeneration of the dryers, for the precooling section as a cooling medium in the chiller tower or simply vented. If there is a need of extra gaseous nitrogen or liquid nitrogen in a facility close to an already existing air separation unit and there is no extra capacity available on this existing asset, one solution is to use the previously mentioned enriched nitrogen stream (“Waste nitrogen”) as a feed to a cryogenic purification unit to produce a gaseous or liquid nitrogen stream at the required purity.

An advantage of certain embodiments of the invention is that there is no need to include a separate purification step as there is no significant carbon dioxide or water amount to be removed. Moreover, the feed is more enriched in nitrogen than air so the distillation is made easier compared with the typical process scheme using air. This waste nitrogen is typically at lower pressure, and therefore, embodiments of the invention include a feed compressor to boost the waste nitrogen feedstock up to a suitable pressure for rectification within the distillation column of the nitrogen generator.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 provides an embodiment of the present invention.

FIG. 2 provides an additional embodiment of the present invention.

FIG. 3 provides yet another embodiment of the present invention.

DETAILED DESCRIPTION

While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims.

In a typical air separation unit, a stream of dry enriched nitrogen (typically above 90%) is coming out of the cold box and is usually used for: (1) the regeneration of the dryers, (2) the precooling section as a cooling medium in the chiller tower, or (3) simply venting to the atmosphere. If there is a need of extra gaseous nitrogen or liquid nitrogen in a facility close to an already existing air separation unit and there is no extra capacity available on this existing asset, one solution is to use the previously mentioned enriched nitrogen stream (“Waste nitrogen”) as a feed to a cryogenic purification unit to produce a gaseous or liquid nitrogen stream at the required purity.

A particular advantage of the invention is that there is no need to include an additional water/carbon dioxide purification step for this enriched nitrogen stream, as there is no significant carbon dioxide or water amount to be removed. Moreover, the feed is more enriched in nitrogen than air, which allows for an easier distillation compared with the typical process scheme using air.

In FIG. 1, air enters an ASU cold box under conditions effective for the rectification of air. In the embodiment shown, the ASU cold box produces gaseous nitrogen, liquid nitrogen, gaseous oxygen, liquid oxygen and liquid argon as products. In addition to these products, the ASU cold box also produces waste nitrogen, which is typically vented or used for other purposes and sent to waste nitrogen users. In the embodiment shown, at least a portion of the waste nitrogen from the ASU cold box can be diverted to a separate nitrogen generator in order to produce a purified gaseous and/or liquid nitrogen product. The nitrogen generator will also produce a small amount of waste nitrogen (e.g., enriched nitrogen stream), which can either be vented or sent to the waste nitrogen users.

FIG. 2 provides a more detailed flow diagram of the nitrogen generator. In this embodiment, the waste nitrogen from the ASU is at a lower pressure as this waste nitrogen stream typically originates from the top of the low pressure column of the ASU, and therefore, the nitrogen stream is compressed in nitrogen compressor before being cooled in a heat exchanger that is separate from the heat exchanger in the ASU. After cooling, the nitrogen is introduced into a single distillation column. Nitrogen vaporizes and accumulates near the top of the column, while other impurities such as oxygen and argon accumulate in the column bottoms. The liquid is removed from the column bottom, expanded across a valve, and then introduced into the top condenser to provide reflux duty (e.g., condense nitrogen in the top condenser coming from the top of the column). The enriched liquid vaporizes in the top condenser and is then warmed in the subcooler and the heat exchanger. To provide additional refrigeration, a portion of this stream can be expanded and then re-warmed in the heat exchanger. The resulting warmed enriched gas is still predominantly nitrogen and can either be vented to the atmosphere or used as a waste gas.

Purified gaseous nitrogen is removed from the top of the distillation column and warmed in the heat exchanger to produce the final gaseous product. If liquid nitrogen is desired, liquid nitrogen can be withdrawn from the condensing nitrogen stream from the top condenser (not shown).

FIG. 3 provides an optional embodiment in which a portion of the enriched nitrogen stream (e.g., waste nitrogen from nitrogen generator) can be recycled to the waste nitrogen compressor in order to further improve the overall recovery of the system. In an additional embodiment, the turbine can be configured to at least partially power the waste nitrogen compressor.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

We claim:
 1. A method for the production of nitrogen, the method comprising the steps of: a) providing a waste nitrogen gas stream at a first pressure called low pressure, wherein the waste nitrogen gas originates from a low pressure column of a cryogenic air separation unit; b) compressing the waste nitrogen gas stream to a second pressure in a waste nitrogen compressor to produce a pressurized nitrogen stream; and c) introducing the pressurized nitrogen stream to a nitrogen generator under conditions effective for producing a purified nitrogen product, wherein the purified nitrogen product has a higher concentration of nitrogen as compared to the pressurized nitrogen stream.
 2. The method as claimed in claim 1, wherein the method further comprises an absence of a purification step for the waste nitrogen gas stream.
 3. The method as claimed in claim 1, wherein the waste nitrogen gas stream consists essentially of nitrogen and impurities, wherein the impurities comprise an absence of water and carbon dioxide.
 4. The method as claimed in claim 1, wherein a portion of the purified nitrogen product is expanded in a turbine to provide cooling for the nitrogen generator.
 5. The method as claimed in claim 1, wherein a portion of the purified nitrogen product is recycled back to the waste nitrogen compressor. 